The invention is related to the field of Nuclear Magnetic Resonance (NMR) apparatus and methods. Specifically, the invention relates to NMR apparatus and methods using pulsed static magnetic fields.
When hydrogen nuclei are placed in an applied static magnetic field, a small majority of spins are aligned with the applied field in the lower energy state, since the lower energy state in more stable than the higher energy state. The individual spins precess about the applied static magnetic field at a resonance frequency also termed as Larmor frequency. This frequency is characteristic to a particular nucleus and proportional to the applied static magnetic field. An alternating magnetic field at the resonance frequency in the Radio Frequency (RF) range, applied by a transmitting antenna to a subject or specimen in the static magnetic field flips nuclear spins from the lower energy state to the higher energy state. When the alternating field is turned off, the nuclei return to the equilibrium state with emission of energy at the same frequency as that of the stimulating alternating magnetic field. This RF energy is generating an oscillating voltage in a receiver antenna whose amplitude and electronic rate of decay depend on the physicochemical properties of the tissue and the magnetic environment of the nuclei. The applied RF field is designed to perturb the thermal equilibrium of the magnetized nuclear spins, and the time dependence of the emitted energy is determine by the manner in which this system of spins return to equilibrium magnetization. The return is characterized by two parameters: T1, the longitudinal or spin-lattice relaxation time; and T2, the transverse or spin-spin relaxation time.
There are at least two applications in which samples volumes are substantial and bulk material properties are of interest. One of these is logging of wells drilled for hydrocarbon recovery from earth formations and another is whole body fat determination.
Measurements NMR parameters of fluid filling the pore spaces of the earth formations such as relaxation times of the hydrogen spins, diffusion coefficient and/or the hydrogen density is the bases for NMR well logging. NMR well logging instruments can be used for determining properties of earth formations including the fractional volume of pore space and the fractional volume of mobile fluid filling the pore spaces of the earth formations.
Pulsed RF magnetic fields are imparted to the material under investigation to momentarily re-orient the nuclear magnetic spins of the hydrogen nuclei. RF signals are generated by the hydrogen nuclei as they spin about their axes due to precession of the spin axes. The amplitude, duration and spatial distribution of these RF signals are related to properties of the material under investigation. In the well logging environment, contrast is high between free and bound fluids based on their relaxation times, between oil and water based on their relaxation times and diffusion coefficient. In medical applications, tissue contrast is high between fat and muscle based on their relaxation times and can be further enhanced by application of certain RF sequences.
Methods of using NMR measurements for determining the fractional volume of pore space and the fractional volume of mobile fluid are described, for example, in Spin Echo Magnetic Resonance Logging: Porosity and Free Fluid Index Determination, M. N. Miller et al, Society of Petroleum Engineers paper no. 20561, Richardson, Tex., 1990. In porous media there is a significant difference in T1 and T2 relaxation time spectrum of fluids mixture filling the pore space. For example, light hydrocarbons and gas may have T1 relaxation time of about several seconds, while T2 may be three orders of magnitude smaller. This phenomenon is due to diffusion effects in the presence of gradients in the static magnetic field. The gradients may be external (from the applied static field) or internal. Internal magnetic field magnitude gradients are due to differences in magnetic susceptibility between the rock matrix of the formation and the fluids in the pores of the matrix.
Power requirements in NMR oil well logging have to be optimized for high efficiency operation. In order to perform a valid NMR experiment, a substance should be polarized for about 5 times the longest T1 relaxation time, which is about 1 second long. Typical Carr-Purcell-Meiboom-Gill (CPMG) pulse sequences are about 0.5 to 1 second long. However, because of low signal-to-noise ratio (SNR), several repetitions of a CPMG sequence are required to bet an adequate SNR.
The earliest NMR logging instruments used the earth""s magnetic field for providing the static field for NMR measurements. See, for example, U.S. Pat. No. 3,004,212 to Coolidge et al; U.S. Pat. No. 3,188,556 to Worthington; U.S. Pat. No. 3,538,429 to Baker; and U.S. Pat. No. 2,999,204 to Jones et al. The earth""s magnetic field is approximately 60 xcexcT at the poles with a Larmor frequency f for protons of approximately 2.5 kHz. The signal level per unit volume for an NMR survey is approximately proportional to f7/4. The early NMR logging instruments suffered from the problem of low resolution because signals from a large volume of the earth were required to get an acceptable SNR. When the earth""s magnetic field is used for the static field, there is no problem in having a uniform static field over a large region, so that SNR is not a major problem; however, there are many applications in which high resolution is required. This is difficult to achieve using the earth""s magnetic field as the static field for NMR experiments.
In order to achieve high resolution, NMR devices used in recent years for well logging operations use permanent magnets to generate the static magnetic field. These devices typically operate at 1 MHz corresponding to a magnetic field in the region of investigation of 0.0235T. Needless to say, this requires the use of permanent magnets with a strong magnetic field as part of the logging instrument.
For example, U.S. Pat. No. 4,350,955 to Jackson et al discloses a pair of permanent magnets arranged axially within the borehole so their fields oppose, producing a region near the plane perpendicular to the axis, midway between the sources, where the radial component of the field goes through a maximum. Near the maximum, the field is homogeneous over a toroidal zone centered around the borehole. U.S. Pat. No. 4,717,877 to Taicher et al teaches the use of elongated cylindrical permanent magnets in which the poles are on opposite curved faces of the magnet. The static field from such a magnet is like that of a dipole centered on the geometric axis of the elongated magnets and provides a region cf examination that is elongated parallel to the borehole axis. The RF coil in the Taicher device is also a dipole antenna with its center coincident with the geometric axis of the magnet, thereby providing orthogonality of the static and magnetic field over a full 360xc2x0 azimuth around the borehole. U.S. Pat. No. 6,023,164 to Prammer discloses a variation of the Taicher patent in which the tool is operated eccentrically within the borehole. In the Prammer device, NMR logging probe is provided with a sleeve having a semi-circular RF shield covering one of the poles of the magnet: the shield blocks signals from one side of the probe.
These, and others too numerous to mention, have been used for wireline logging wherein the logging tool is conveyed on a wireline into a borehole, as well as Measurement-While-Drilling (MWD) operations where the logging tool forms part of the drilling assembly. All of these tools typically have a region of investigation no more than a few centimeters into the formation and a few millimeters in thickness. Repeatability of the observations requires that the static magnetic field be predictable to a high level of accuracy. An unappreciated problem in NMR logging of earth formations using strong permanent magnets is that the static magnetic field in the subsurface may not correspond to that expected on the basis of the design of the magnet. This is due to the fact the logging instruments, whether on a wireline or as part of an MWD apparatus, have to pass through several hundreds or thousands of meters of casing that is used to line boreholes. To understand the consequences of this, a brief review of the process of drilling wells is needed.
In the drilling of oil and gas wells, drill bits and other equipment are attached to a drill string for boring a hole into the earth. Typically, a drill string may comprise a long string of many connected sections of drill pipe which extend from the earth""s surface down into the wellbore or hole being formed by a drill bit connected at the bottom end of the drill string. As the wellbore penetrates more deeply into the earth, it becomes increasingly desirable to install casing in the wellbore, running down from the surface.
Casing is placed in the wellbore for one of two reasons. The first may be to prevent the wall of the wellbore from caving in during drilling and to prevent seepage of fluids from the surrounding strata into the wellbore. Casing is absolutely essential when drilling through an overpressured section (with an abnormally high fluid pressure requiring heavy drilling muds) into a normally pressured or underpressured section below: in such situations, casing is set after drilling through the overpressured formation and the mud weight is reduced. A second reason may be to prevent damage to the reservoir rocks by the drilling mud in the borehole forcing its way into the formation. Even in normal drilling, it is common to set casing of several different sizes in the borehole.
During rotary drilling operations drill strings are subjected to shock, abrasion and frictional forces which are exerted on the drill string whenever the drill string comes in contact with the walls of the wellbore or casing. Both the drillstring and the casing are usually made of steel, a ferromagnetic material, so that the abrasion forces will result in large quantities of ferromagnetic debris within the casing. There are numerous methods and devices for reducing the abrasion. None of them can be completely effective. Circulating drilling mud during drilling is quite effective in bringing cuttings from the formation to the surface but is not effective in completely flushing the more dense metallic debris out of the borehole.
As a result of this, when an NMR logging tool, whether on a wireline or as part of an MWD apparatus, is conveyed into a borehole through casing, much of the magnetic debris within the casing will attach to the tool. This can distort the static magnetic field produced by the permanent magnets in an unpredictable manner. In addition, since the RF pulses are produced by transmitter coils on the logging tool, the RF field is also distorted. Compounding the problem is the fact that the spin-echo signals also have to pass through this debris. U.S. Pat. No. 5,451,873 to Freedman et al. teaches a method of calibrating an NMR tool to account for the accumulation of magnetic debris on the tool. For a so-called xe2x80x9csaddle pointxe2x80x9d tool used in Freedman, one effect of the debris is to change the static field (and hence the Larmor frequency) in the region of investigation. Freedman makes a one-time adjustment to the tool frequency prior to using the tool. The frequency shift is not necessary for gradient tools since for a fixed frequency, the volume of investigation changes. A continuing problem remains: how to compensate for time varying effects of the debris.
In addition to the signal distortion, there is also the practical problem of conveying a strongly magnetized logging tool several meters long through a ferromagnetic casing. This problem is exacerbated in deviated or horizontal boreholes.
In one embodiment, the present invention is a method for nuclear magnetic resonance (NMR) sensing of earth formations. An electromagnet on a logging tool is used to induce a static magnetic field for polarization of nuclei within a region of the earth formations. A radio frequency pulse is used to tip the magnetic spins of the nuclei. A receiver is used to measure either the free induction decay or spin echo signals (using a CPMG pulse sequence) from the precessing nuclei. The wait time between the activation of the electromagnet and the initial RF pulse is related to a T1 of the formations. When the static magnetic field strength is 10-100 times that of the earth""s field, it is possible to obtain low resolution estimates of properties of large volumes of earth formation. The logging tool may be conveyed into the earth on a wireline or on a drilling tubular.
In an alternate embodiment of the invention, a time varying static field is produced using an electromagnet. The transmitter and the receiver operate at different frequencies. This reduces the ringing signals in the receiver and, after calibration, provides a measurement of bulk composition
Another embodiment of the invention may be used for estimating fat composition of a human body. A prior art MRI device is operated in accordance with a method of present invention to provide a low intensity static magnetic field, making it possible to obtain low cost body fat and lean measurements.